Control method and apparatus for the production of powder metal

ABSTRACT

Control of the process for producing pre-alloyed metallic powder by controlling the melting rate of at least one consumable electrode formed from an ingot of the metal to be ultimately atomized, to produce a controllable continuous flow of metal for atomization into powder. After striking an arc for melting the electrode which is located in a chamber containing a protective atmosphere, the power input to the electrode is selectively adjusted during continual melting to control the melting rate to equal the optimum rate of atomization. The liquid metal droplets from the electrode are collected in a preheated holding reservoir to provide a homogeneous stream of metal which is delivered to a disintegrator. This produces an atomized liquid metal which solidifies into a powdered form and is collected.

Hui-ted States Patent [191 Marmara et a1.

[ 1 Aug. 13, 1974 [75] Inventors: Falih N. Darrnara; I. Dwight Clarlt,

both of New Hartford, NY.

[73] Assignee: Special Metals Corporation, New Hartford, NY.

[22] Filed: Oct. 3, 1972 [21] Appl. No.: 294,747

Primary ExaminerJ. Spencer Overholser Assistant Examiner.10hn E. Roethel Attorney, Agent, or FirmVincent G. Gioia; Robert F. Dropkin [5 7 ABSTRACT Control of the process for producing pre-alloyed metallic powder by controlling the melting rate of at least one consumable electrode formed from an ingot of the metal to be ultimately atomized, to produce a controllable continuous flow of metal for atomization into powder. After striking an arc for melting the electrode which is located in a chamber containing a protective atmosphere, the power input to the electrode is selectively adjusted during continual melting to control the melting rate to equal the optimum rate of atomization. The liquid metal droplets from the electrode are collected in a preheated holding reservoir to provide a homogeneous stream of metal which is delivered to a disintegrator. This produces an atomized liquid metal which solidifies into a powdered form and is collected.

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BACKGROUND OF THE INVENTION The present invention relates to a method of control and control apparatus for the production of metallic powder using ingots of pre-alloyed metal as consumable electrodes.

Certain prior practices for producing metallic powder from a consumable electrode involve striking an are between a metallic consumable spinning electrode and a tungsten electrode; while other practices involve the striking of an arc between a metallic consumable electrode and a spinning wheel. A particular prior art practice is disclosed in US. Pat. No. 2,897,539 issued on Aug. 4, 1959. The process disclosed therein is disadvantageous insofar as it requires a starter body to protect the rotating table from damage caused by the arc and to preclude contamination of the powder being produced. In addition, it is plagued by a troublesome sliding electrical contact to the rotating table.

Another prior art process is disclosed in US. Pat. No. 2,310,635 issued Feb.-9, 1943. This process is viewed as critically deficient to provide a homogeneous and uniform liquid metal composition throughout a given period of time due to the necessity of feeding raw material components through a hollow electrode. In other words, a pre-alloyed ingot is not used. Moreover, this process is plagued with trouble surrounding the use of electroslag as a means for avoiding contamination of l the liquid metal; and it is troubled with a sliding electrical connection between the power supply and the hollow electrode.

Many prior art consumable electrode melting processes for producing metallic powder have been found critically deficient in the aspect of controlling the process for producing metallic powders and particularly control of the solidification of the atomized metal. In known processes, the liquid metal from electrode melting is collected in a holding chamber. This is particularly desirable for the production of pre-alloyed, superalloy metallic powder since superalloy consumable electrodes have a segregated; i.e., heterogeneous, makeup. Superalloy consumable electrodes have a concentration of low melting point constituents, elements and compounds in their center and form metal droplets of varying composition when melted. The grains of the consumable electrode grow inwardly during their solidification and drive the low melting point constituents, elements and compounds along with them, thereby setting up a degree of segregation within the electrodes. This degree of segregation is more prominent in superalloys although troublesome with many other alloys used in powder metallurgy. The lack of homogeneous properties. in the liquid metal droplets has been overcome by providing a shallow reservoir for holding a volume of molten metal sufficient to provide a metallurgically homogeneous metal stream which is then delivered to an atomizing station for the production of the metallic powder. While this process has greatly improved the production of metallic powders by providing greater uniformity of the constituent elements in the powder, it has been found that there is a great difficulty in providing a controlled, continuous slow flow of metal to the atomization apparatus. It is highly desirable to be able to relate the melting rate of the ingot to a given optimum rate of atomization of the metal powder.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus for producing metallic powder by an electrode melting process in which molten metal is supplied as a continuous controllable stream even at relatively slow flow rates, to an atomization apparatus. In one aspect, the control is based on adjustments of the power input to two consumable electrodes to control the melting rate thereof. Concurrently, adjustment is made to the position of the electrodes to maintain an arc gap for continuous melting of the electrode. This can be accomplished, for example, by an arc voltage control system.

According to the invention, there is provided a method of producing pre-alloyed metallic powder comprising the steps of providing at least one electrode having a composition corresponding to the desired metallurgical composition of the powder to be ultimately produced, striking an arc to melt the consumable electrode in a controlled atmosphere and without the formation of any substantial slag, controlling the rate of continuous melting of the consumable electrode by power input adjustments on the basis of a desired flow rate of the molten metal to the atomization means, collecting the molten metal in a reservoir, holding the mo]- ten metal in the reservoir for a period of time sufficient to homogenize it, atomizing the homogenized metal passing from the reservoir, and cooling the atomized metal in a chamber having a controlled atmosphere. The method according to the present invention additionally provides the step of adjusting the arc gap between the electrodes to maintain desired arc conditions therebetween for continuous melting of the consumable electrode.

An important feature of the invention is to provide a system for controlling the melting rate in a consumable electrode melting process wherein the melting rate is very accurately controlled and adjusted to match a predetermined flow rate of the molten metal supply to the atomization means by adjustments of the power inputs to the consumable electrodes. An electrode gap adjustment circuit, based on arc voltage or the like, may be concurrently operated to maintain the gap between electrodes at a desired dimension to provide a continuous delivery of liquid metal droplets from the consumable electrode.

These features and advantages of the present invention as well as others will be more readily understood when the following description is read in light of the accompanying drawings, in which:

FIG. 1 is a schematic view of one embodiment of the melting means of the powder making system of the present invention;

FIG. 2 is a schematic view of another embodiment of the melting means of the powder making system of the invention; and

FIG. 3 is a schematic view of the preferred embodiment of the invention including the preferred melting means, atomization means and powder collecting means.

With reference now to FIG. 1, there is provided a tundish 10 having a heating source (e.g., an induction coil 11) for preheating the tundish to prevent premature freezing of liquid metal passing into it from consumable electrode 52 and fluid cooled non-consumable stool 54. The tundish also includes a skimmer wall 14 to prevent the outflow of impurities which may be floating on the metal surface in the tundish. Molten metal passes from'the tundish through a runner channel 15 to the atomization apparatus, which in the embodiment shown (FIG. 3) is a rotating drum (hereinafter discussed).

With reference now to electrode 52, in the preferred form, this electrode is formed from an ingot of prealloyed metal corresponding in composition to that of the ultimate powder to be produced. Below this electrode is a non-consumable fluid cooled stool 54. In this embodiment, the electrode and stool are connected by transmission lines and 31, respectively, to a power supply 32 and potentiometer 33 having a slide wire 34 which is manually set to vary the power delivered from the supply 32 to the electrode and the stool. The electrode 52 is contained within an evacuated chamber 51 within which it is supported vertically by means of supp'ort rod assembly 53. This support rod assembly 53 permits adjusting the position of the electrode to strike an are on the non-consumable stool 54. As liquid metal droplets fall from the consumable electrode 52, they form a shallow pool on the stool 54 from where the liquid metal flows into a preheated tundish 10. The tundish can be heated by electrical induction or resistance or other means 11. As the metal passes from the tundish, a skimmer wall 14 prevents the outflowing of slag or other impurities whichmay float upon the surface of the metal. The runner directs the liquid metal to the atomization apparatus.

FIG. 2 illustrates the same embodiment of FIG. 1, except for a different runner or pouring nozzle 59 which receives molten metal from stool 58 and directs the molten metal to the atomization apparatus. Pouring nozzle 59 may be heated by means of the electrical resistance or induction coils 60. v

The use of the stool 54 in the embodiment of FIG. 1 and stool 58 according to the embodiment of FIG. 2 provides an immediate shallow pool area for collecting liquid metal droplets from the electrodes. The depth of the pool is selected for optimum temperature of the metal flowing from the stool. This depth is usually less than one-third the diameter of the electrode 52.

With reference now to FIG. 3, an embodiment is shown which includes two consumable electrodes 12 and 13 in the preferred form. These electrodes are formed from ingots of pre-alloyed metal corresponding in composition to that of the ultimate powder to be continuously produced. The electrodes are connected by electrical transmission lines 30 and 31, respectively, to a polarity reversing switch 71 and through other electrical transmission lines 73 and 74, respectively, to power supply 32. A potentiometer 33 having a slide wire 34 is manually set to vary the power delivered from the supply 32 to the electrodes. A sight glass 35 1 is provided in the housing for viewing the arc gap between the electrodes which are adjusted to continually maintain a predetermined arc gap by hydraulic drives 36 and 37. Lines 38a and 38b connect the hydraulic drives to a servo system which is controlled by an electrode position control 39 through the lines 40a and 46b. Electrodes l2 and 13 are oscillated at least about 180 in opposite directions by means of motors 69 and 70. Oscillation speed is controlled through a timer, not

shown, at a preset desired rate. A theoreticalalternative to oscillation is rotation ,of electrodes in opposite directions. However, at the present time oscillation has proven to be more practical.

The apparatus is placed in operation by adjusting the relative position of the consumable electrodes 12 and 13 through the operation of hydraulic drives 36 and 37 to obtain proper spacing for striking an electric arc. Current passes through the lines 30 and 31 in order to strike an arc and melt the electrodes. Proper spacing and centering of the electrodes is maintained by the electrode position control 39 as the electrodes are continuously consumed. The electrodes 12 and 13 are continually oscillated by means of motors 69 and 70 to insure uniform burn-off across the opposing faces of the electrodes during the melting process. The character of the arc is observed through the sight ports 35. In order to further compensate for the problem of unequal burn-off rates of electrodes with opposite polarities, the polarities of the electrodes are preferably changed periodically by means of polarity reversing switch 71. Mo]- ten metal from the electrodes is collected to form a pool of liquid metal and homogenize it in tundish 10, which may be similar to that described in the embodiment of FIG. 1. The metal from tundish 10 flows under skimmer 14 and through nozzle 15 onto the atomizing wheel or drum 16. The remainder of the process is identical with that described in the embodiment of FIG.

Turning, now, to the presently preferred atomization means, as shown in FIG. 3, the metal discharged by the runner channel 15 in a continuous and controlled flow impinges upon the outer surface of a drum 16 having bearing mounted shafts 16a extending horizontally and connected to a drive motor 1612. It is preferred to form the peripheral surface of the drum with projecting teeth or vanes which are water cooled and which may be replaceable. As the stream of liquid metal impinges upon the surface of the drum, it is atomized into a spray of liquid metal which passes through a movable shield 17 into a chamber 18 where the metal solidifies into a powdered form that is collected on a transfer surface of a vibrator conveyor 19. The chamber 18 as well as the space surrounding electrodes 12 and 13 and tundish 10 is evacuated or otherwise provided with a controlled non-oxidizing atmosphere. The temperature of the powdered metal on the conveyor must fall below a temperature at which there occurs a latent heat of fusion and superheat of the solidified metal so as to avoid impairing the quality and particle size of the metal powder. This is accomplished by controlling the supply and flow of liquid metal through potentiometer 33 as aforesaid, entering the holding chamber II. In order to monitor the quantity and other characteristics of atomized metal entering the chamber 18, a sight glass 21 is provided in the end wall of the chamber for viewing the stream of metal discharged from the runner 15 onto the peripheral surface of the drum 16.

The volume of liquid metal entering the holding chamber may be adjusted and controlled according to the stream of metal flowing from the runner 15 and to coincide with the optimum atomization rate for the equipment used.

In place of the rotating drum 16, a disc may be used which is arranged for receiving a stream of metal on its upper horizontal surface and atomizing it. In still another form, a plasma gun may be used to atomize the liquid metal or an. argon nozzle may be used. In all cases given aboveboth the electrodes and the atomization means are in a vacuum or in a controlled nonoxidizing atmosphere.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

What is claimed is:

l. A method of producing pre-alloyed metal powder with atomization means comprising the steps of,

providing at least one electrode having a composition corresponding to the desired metallurgical composition of the metal powder to be ultimately produced,

striking an arc to melt said consumable electrode in a controlled atmosphere and without the formation of any substantial slag, controlling the rate of continuous melting of said consumable electrode by adjusting the power input to the electrode as a function of desired flow rate of molten metal to, said atomization means,

collecting the molten metal from said consumable electrode in a reservoir,

holding the molten metal in said reservoir for a period of time sufficient to form a homogenized stream of liquid metal,

atomizing said homogenized stream of liquid metal passing from said reservoir, and v cooling said atomized metal in a chamber having a controlled atmosphere.

2. The method according to claim 1 comprising the additional step of adjusting the gap between two electrodes to maintain an arc therebetween.

3. The method of claim 2 including the step of rotating at least one of said electrodes about its axis to insure uniform burn-off during the melting process.

4. The method of claim 3 wherein both electrodes are oscillated about their axes.

5. The method according to claim 2 wherein said striking an arc produces liquid metal from a second electrode form from an ingot corresponding to the desired metallurgical composition of the metal powder to be ultimately produced.

6. The method according to claim 1 wherein said atomizing comprises the step of passing said homogenized stream of liquid metal onto a moving disintegra- I01.

7. The method according to claim 1 wherein said atomizing comprises the step of passing said homogenized stream of molten metal into an inert gas stream.

8. The method according to claim 1 wherein said atomizing comprises the step of passing said stream of homogenized liquid metal into a stream from a plasma gun.

9. An apparatus for producing pre-alloyed metal powder comprising,

an enclosed chamber having a controlled nonreactive atmosphere therein,

first and second electrodes forming a gap within said enclosed chamber for striking an arc therein, at least one of said electrodes being an ingot of prealloyed metal corresponding to the composition of the pre-alloyed metal powder to be ultimately formed,

a power supply connected to said electrodes to melt the electrode of pre-alloyed metal without the formation of any substantial slag,

electrode position control means including a drive for adjusting said gap to maintain an are between said electrodes,

a liquid metal holding chamber in said enclosed chamber for collecting and homogenizing the mo]- ten pre-alloyed metal from at least one of said electrodes, said holding chamber providing a continuous stream of said homogenized molten pre-alloyed metal,

means for atomizing said continuous stream of molten metal delivered from said liquid metal holding chamber,

means for adjusting the power delivered from said supply to said electrodes to control the supply rate of molten pre-alloyed metal delivered to said means for atomizing, and

means in said housing for collecting metal powder produced upon solidification of said atomized liquid metal.

10. An apparatus according to claim 9 wherein said means for adjusting the power comprises a manually adjustable rheostat.

11. An apparatus according to claim 10 wherein said electrode position control means receives a signal proportional to the power delivered from said supply to said electrodes to maintain continuous melting thereof. 

1. A method of producing pre-alloyed metal powder with atomization means comprising the steps of, providing at least one electrode having a composition corresponding to the desired metallurgical composition of the metal powder to be ultimately produced, striking an arc to melt said consumable electrode in a controlled atmosphere and without the formation of any substantial slag, controlling the rate of continuous melting of said consumable eleCtrode by adjusting the power input to the electrode as a function of desired flow rate of molten metal to said atomization means, collecting the molten metal from said consumable electrode in a reservoir, holding the molten metal in said reservoir for a period of time sufficient to form a homogenized stream of liquid metal, atomizing said homogenized stream of liquid metal passing from said reservoir, and cooling said atomized metal in a chamber having a controlled atmosphere.
 2. The method according to claim 1 comprising the additional step of adjusting the gap between two electrodes to maintain an arc therebetween.
 3. The method of claim 2 including the step of rotating at least one of said electrodes about its axis to insure uniform burn-off during the melting process.
 4. The method of claim 3 wherein both electrodes are oscillated about their axes.
 5. The method according to claim 2 wherein said striking an arc produces liquid metal from a second electrode form from an ingot corresponding to the desired metallurgical composition of the metal powder to be ultimately produced.
 6. The method according to claim 1 wherein said atomizing comprises the step of passing said homogenized stream of liquid metal onto a moving disintegrator.
 7. The method according to claim 1 wherein said atomizing comprises the step of passing said homogenized stream of molten metal into an inert gas stream.
 8. The method according to claim 1 wherein said atomizing comprises the step of passing said stream of homogenized liquid metal into a stream from a plasma gun.
 9. An apparatus for producing pre-alloyed metal powder comprising, an enclosed chamber having a controlled non-reactive atmosphere therein, first and second electrodes forming a gap within said enclosed chamber for striking an arc therein, at least one of said electrodes being an ingot of pre-alloyed metal corresponding to the composition of the pre-alloyed metal powder to be ultimately formed, a power supply connected to said electrodes to melt the electrode of pre-alloyed metal without the formation of any substantial slag, electrode position control means including a drive for adjusting said gap to maintain an arc between said electrodes, a liquid metal holding chamber in said enclosed chamber for collecting and homogenizing the molten pre-alloyed metal from at least one of said electrodes, said holding chamber providing a continuous stream of said homogenized molten pre-alloyed metal, means for atomizing said continuous stream of molten metal delivered from said liquid metal holding chamber, means for adjusting the power delivered from said supply to said electrodes to control the supply rate of molten pre-alloyed metal delivered to said means for atomizing, and means in said housing for collecting metal powder produced upon solidification of said atomized liquid metal.
 10. An apparatus according to claim 9 wherein said means for adjusting the power comprises a manually adjustable rheostat.
 11. An apparatus according to claim 10 wherein said electrode position control means receives a signal proportional to the power delivered from said supply to said electrodes to maintain continuous melting thereof. 